U.S. defense leaders expressed caution Tuesday about military intervention in Libya, warning that enforcement of a no-fly zone would require scarce air assets, domestic political approval and international authorization.

Foreign leaders, and some U.S. officials, have said a no-fly zone is under active consideration, and Defense Secretary Robert M. Gates said the Pentagon was preparing "a lot of options and contingencies" for President Obama.

But Gates said military measures could have indirect consequences that "need to be considered very carefully." He suggested any intervention in Libya could drain U.S. forces from the war in Afghanistan and questioned the wisdom of the United States engaging in military action in another Muslim country.

Gates and Adm. Mike Mullen, the chairman of the Joint Chiefs of Staff, told reporters that they had no confirmed reports that Libyan leader Moammar Gaddafi had used airstrikes against civilians or opposition forces that occupy the eastern half of the country. Gates also said there had been no request by the rebels for military assistance.

But opposition leaders in the eastern city of Benghazi, frustrated by their inability to dislodge Gaddafi from his stronghold in Tripoli, said they were considering making such a request.

"We are well aware of the ongoing efforts by Col. Gaddafi to defend the area of Tripoli and a few other places that he continues to hold," Secretary of State Hillary Rodham Clinton told U.S. lawmakers Tuesday. The opposition, she said, has been "working to create more of a military presence so they can not only defend places already taken over but try to take territory away" from Gaddafi.

Former Air Force generals said that while a no-fly zone could be set in motion relatively quickly, enforcement would require hundreds of fighter jets and support aircraft as well as a coordinated bombing campaign.

The Libyan air defense system is significantly more advanced than the Iraqi defenses that the U.S.-led force destroyed in 2003 in the early stages of that invasion. "It consists of much more modern surface-to-air missiles," said Lt. Gen. David Deptula, a former fighter pilot who until recently oversaw Air Force intelligence in the Pentagon.

The administration has sharply increased both its rhetoric and actions against Gaddafi in recent days, hoping to persuade senior Libyan military and regime leaders to decide that his cause is lost and that they should turn against him. At the same time, it is coordinating possible military actions with European allies in case they become necessary.

As it seeks to calibrate its message, the administration is also trying to determine who is in charge of the opposition. With few direct contacts in Libya, U.S. officials have queried those with business and other non-governmental ties for names, phone numbers and assessments of those who appear to be in charge.

"It's a very fluid situation, but we are reaching out through these different channels to a variety of people who are in the opposition," White House spokesman Jay Carney said. "We want to hear from and learn from and talk to those who have a desire to move towards a representative government."

Clinton, in testimony Tuesday on the administration's foreign policy budget for fiscal 2012, referred to intense administration activity on Libya as an example of the kind of capabilities that would be undermined by budget cuts proposed by House Republicans.